The present invention relates to a laser based system for gaging the outside dimension of an object and, more particularly, to a laser based gaging system using imaging and interferometry.
Laser based gaging systems are generally known in the art and are useful in situations where high accuracy is required or where an object to be measured cannot be touched because it is, for example, sensitive, hot, toxic, or the like.
Optical gaging systems are grouped into two major categories. The first category includes systems using imaging or measurement of light reflected from an object to be measured or from a background surface. Such systems include electronic camera based vision systems, beam scanning systems and optical comparators, and profiling systems using front or oblique lighting. The second category includes systems using imaging or measurement of light transmitted past, and not "eclipsed", by the object to be measured. Systems in the second category include gaging microscopes, optical comparators using rear lighting methods, and beam scanning systems using beam interruption methods.
A search of the prior art failed to uncover any prior art reference which discloses the laser based gaging system of the present invention. However, several prior art patents were uncovered which disclose various laser based optical gaging systems.
For example, U.S. Pat. Nos. 3,856,412 and 4,199,259 disclose optical gaging systems wherein a laser beam is scanned by a rotating mirror across a beam splitter, from which one half of the beam is used to calibrate the system and the other half of the beam is eclipsed by a stationary object to be measured. The eclipsed beam is projected onto a photocell which produces a signal from which the diameter of the object is calculated.
U.S. Pat. No. 4,063,103 discloses an electron beam exposure apparatus wherein a pattern is formed on a workpiece disposed on a carriage movable in both X and Y directions. A laser interferometer is used to measure the distance over which the carriage moves.
U.S. Pat. Nos. 3,765,774; 3,905,705; and 4,129,384 disclose optical gaging systems wherein a laser beam is scanned by a rotating mirror across a stationary object. The beam is eclipsed by the object and imaged onto a photodetector. The detector signal is then used to calculate the object size.
U.S. Pat. No. 3,743,428 discloses an optical measuring device wherein a laser beam is eclipsed by a stationary test object and then subjected to a grid to divide the beam into a series of pulses which are imaged by a rotating scanning mirror onto a slit and photodetector. The size of the test object is calculated by counting the number of pulses that pass through the slit.
The above-described prior art devices have a number of common disadvantages which limit precision. Those devices have a large depth of field that limits their accuracy due to blurring of the detected edge. In addition, they are affected by the varying amount of the light beam that grazes the object. Furthermore, their beam eclipse signatures are as wide as the scanning beam. Such systems also require an accurate angular-to-linear scan conversion lens.
U.S. Pat. Nos. 3,765,774 and 3,905,705 disclose devices the accuracy of which is limited by errors in alignment and angular velocity of the scanning mirrors.
U.S. Pat. Nos. 3,856,412 and 4,199,259 are limited by the accuracy and stability of their calibration scales and gratings.
U.S. Pat. No. 4,129,384 describes a dual beam method for measuring object dimensions. The accuracy of that method depends on factors associated with the beams, which are required to be of equal energy, co-aligned and highly symmetric. Additional disadvantages of the dual beam method are its complexity and high cost.